1. Field of the Invention
The present invention relates to an ignition timing control device for an engine having an idle speed control device for controlling the engine speed during an idle operation.
2. Description of the Related Art
An idle speed control device is used to maintain an engine speed at a predetermined target value during the idle operation regardless of changes in engine temperature and engine load. The idle speed control device is usually equipped with an inlet air bypass passage connecting the portions of the inlet air passage upstream and downstream of the throttle valve, and an idle speed control valve for controlling the airflow passing through the inlet air bypass passage. The idle speed control device adjusts the engine speed by controlling the amount of the inlet air supplied to the engine using the idle speed control valve, regardless of the degree of opening of the throttle valve, during the engine idle operation.
However, since the idle speed control device supplies inlet air regardless of the opening of the throttle valve, an operator of the engine cannot control the amount of inlet air by operating the throttle valve if the idle speed control device fails. Therefore, if the idle speed control is locked at its fully opened position due to the failure of the idle speed control device, an over-running in which the engine speed increases excessively, against the operator's wishes, may occur.
In order to prevent such over-running of the engine, various control devices have been proposed. For example, Japanese Unexamined Patent Publication (Kokai) No. 60-156979 discloses an ignition timing control device which retards the ignition timing of the engine in accordance with the engine speed when the speed of the engine in an idle operation exceeds a predetermined value. Since the engine output torque decreases as the ignition timing is retarded, the engine speed can be reduced by retarding the ignition timing. In the device in JPP'979, the over-running is prevented by retarding the ignition timing even if the idle speed control is locked at its fully opened position due to a failure of the idle speed control device.
However, the ignition timing control device in JPP'979 performs the ignition timing retarding operation only when the engine speed exceeds a predetermined value. This may cause problems in some cases. For example, the engine idle speed varies largely when the engine load changes. Therefore, when relatively large load is exerted on the engine, for example, when an air conditioner is ON, the engine idle speed does not increase excessively even if the idle speed control valve is locked at its fully opened position. Since the device in JPP'979 does not retard the ignition timing when the engine speed is lower than a predetermined value, if the air conditioner is ON, the ignition timing is not retarded even when a failure of the idle speed control device occurs. In this case, if the air conditioner switch is turned off, the engine speed suddenly increases and a temporary over-running occurs until the ignition timing is retarded to an appropriate value.
Further, the device in JPP'979 performs the retarding operation of the ignition timing only when the valve is fully closed. However, if the idle speed control valve is locked at its fully opened position due to a failure of the idle speed control device, over-running of the engine can occur even when the engine is operated in a low load condition (i.e., when the throttle valve is not fully closed) in the actual operation of the engine. Therefore, it is difficult to prevent the over-running of the engine if the ignition timing retarding is performed only when the throttle valve is fully closed.
In order to prevent over-running of the engine during low load operation of the engine, the ignition timing may be retarded in accordance with the engine speed when the throttle valve is opened. However, if the ignition timing retarding operation is performed regardless of the engine load when the throttle valve is opened, other problems may occur. For example, when the engine is operated at a relatively high load, the engine operation is not affected even if the idle speed control valve is locked at its fully opened position due to the failure of the idle speed control device since the amount of the inlet air of the engine is relatively large in a high load operation and the increase in the amount of inlet air caused by the failure of the idle speed control device is small when compared with the total amount of inlet air of the engine. Therefore, when the engine is operated in a relatively high load, it is not necessary to retard the ignition timing even if the idle speed control device fails. Further, if the ignition timing is retarded in such conditions, a shortage of the engine output torque and an excessive temperature rise of the exhaust gas may be caused by the ignition timing retarding. Therefore, it is not preferable to perform the ignition timing retarding when the engine operating load is relatively high even when a failure of the idle speed control device occurs.